confused by medieval manuscripts

I found the following from a website on Medieval Church music:

Non dico tibi petre dimittendi septies sed usque septuagies septies

Based on the context, the translation should be something like: I do not say to you, Peter, to forgive seven times, but seventy seven times. My question is how dimittendi, which looks like either a gerund or a gerundive in the genitive, could be grammatically correct. If it were only in one manuscript, I might argue that it’s just a scribal error. But it’s in a series of manuscripts.
Can anyone help me figure this out? Thanks.

It would be gerund (verbal noun), not gerundive (verbal adjective). A Google search indicates that it was a different reading of Mat. 18:22.
E.g., https://books.google.com/books?id=uCFiAAAAcAAJ&pg=PA666&lpg=PA666&dq=Non+dico+tibi+petre+dimittendi+septies+sed+usque+septuagies+septies&source=bl&ots=BNpfPOqQb9&sig=3JrcKLV2fX0T684lag3HJVMAo-k&hl=en&sa=X&ved=0ahUKEwjb2PTfqOjTAhWnjlQKHYAeDhQQ6AEISTAF#v=onepage&q=Non%20dico%20tibi%20petre%20dimittendi%20septies%20sed%20usque%20septuagies%20septies&f=false , https://books.google.com/books?id=2fVQAQAAMAAJ&pg=PA1563&lpg=PA1563&dq=Non+dico+tibi+petre+dimittendi+septies+sed+usque+septuagies+septies&source=bl&ots=ZZHpC1-gpD&sig=PJUmrmMkFC_-nSV0bDSnJK61I9c&hl=en&sa=X&ved=0ahUKEwjb2PTfqOjTAhWnjlQKHYAeDhQQ6AEIUTAH#v=onepage&q=Non%20dico%20tibi%20petre%20dimittendi%20septies%20sed%20usque%20septuagies%20septies&f=false , https://books.google.com/books?id=uCFiAAAAcAAJ&pg=PA666&lpg=PA666&dq=Non+dico+tibi+petre+dimittendi+septies+sed+usque+septuagies+septies&source=bl&ots=BNpfPOqQb9&sig=3JrcKLV2fX0T684lag3HJVMAo-k&hl=en&sa=X&ved=0ahUKEwjb2PTfqOjTAhWnjlQKHYAeDhQQ6AEISTAF#v=onepage&q=Non%20dico%20tibi%20petre%20dimittendi%20septies%20sed%20usque%20septuagies%20septies&f=false

In the Office and in the Mass there are many chants whose text does not correspond to St. Jerome’s Vulgate. When the Vulgate became the official translation of the Latin Church, that applied only to the lessons and psalmody (but some places retained the older usages for centuries). The chants were left untouched.

Anyway, it doesn’t look like Ciceronian Latin to me.

Gerundive would almost give a nice meaning: dimittendum [est] ‘it is to be forgiven’ (in Classical Latin this verb is not used in the sense ‘to forgive’). As it isn’t neuter singular nom-acc, it can’t be quite like this. Can a suitable word be added thereto? Not far off would be a substantivised dimittendus used in plural along with a numeral. It still isn’t quite satisfactory though.

There actually was at least one manuscript that did read ‘dimittendum,’ as if to correct the prevailing tradition. And might not that make more sense? What if it were referring to the sinner in the previous sentence of Matt. 18? Couldn’t you then read it as the accusative subject of an indirect statement, the verb esse being understood?

Dimittendum makes sense. [est]/[esse] can very easily be understood. ‘It has to be forgiven 77 times’, ‘One has to forgive 77 times’ or something like that. Acc.c.inf. is probably best (esse understood), though direct speech is not impossible (with a colon and est understood).

Maybe there was some kind of contamination with Mt. 9:6 (cfr. Mc. 2:10, Lc. 5:24)?

Ut autem sciatis, quia Filius hominis habet potestatem in terra dimittendi peccata, tunc ait paralytico : Surge, tolle lectum tuum, et vade in domum tuam.

Bedwere and Timothée:
Even though it would have been nice to find some justification for the strange sentence, it’s also nice to know that my confusion was, in this case, not due to my insufficient command of the language. It remains curious, however, that almost all of the manuscripts that I found contain the error. How could so many copyists be oblivious to it? Unless, being monks, they were just copying in the spirit of obedience!

At any rate, thank you so much for assisting me with this.

Placing the key parts in direct speech seems to me to resolve the difficulty:
Non dico tibi, Petre, “dimittendi septies” sed “usque septuagies septies”.

Very clever. However, shouldn’t it be [peccata] dimittenda? Your interpretation seems to say that [peccatores] dimittendi, the sinners must be sent away.

Actually, that may work. Stelton’s Dictionary of Ecclesiastical Latin includes forgive among the possible meanings of dimittere. So the passage would translate:

Do not say, Peter, “He [the sinner] should be forgiven seven times,” but " up to seventy seven times."

Ingenious.

Of course it means forgive. The problem is that it takes the dative of the person forgiven, not the accusative. E.g., dimitte nobis debita nostra.

The person forgiven isn’t mentioned, surely? If we now add the understood [sunt], doesn’t it work?

I don’t think so.
I can find plenty of evidence of “dimittenda peccata” in the sense of sins are to be forgiven.
I can find no evidence of “dimittendi peccatores” in the sense of sinners are to be forgiven
Dimittendi peccatores means that sinners are to be sent away, not forgiven.
E.g., https://books.google.com/books?id=v331Wl7Uzu8C&pg=PA755&lpg=PA755&dq=dimittendi+peccatores&source=bl&ots=T3bzf_KBHh&sig=gAEGKXSlCqBE5S7rJLKywSCYLSM&hl=en&sa=X&ved=0ahUKEwihtNPk3OrTAhUC0GMKHcayBSEQ6AEIMTAC#v=onepage&q=dimittendi%20peccatores&f=false
https://books.google.com/books?id=lGBnAAAAcAAJ&pg=PA762&lpg=PA762&dq=dimittendi+peccatores&source=bl&ots=0X5PaUBK-f&sig=3GkQ5XksJovOjegxayF9pDigUyI&hl=en&sa=X&ved=0ahUKEwihtNPk3OrTAhUC0GMKHcayBSEQ6AEINzAD#v=onepage&q=dimittendi%20peccatores&f=false
https://books.google.com/books?id=G2FqZDP5Pr4C&pg=PA372&lpg=PA372&dq=dimittendi+peccatores&source=bl&ots=CHswMUK0IK&sig=54eH4LnDp2m-8cNNOH64EyzsHKU&hl=en&sa=X&ved=0ahUKEwihtNPk3OrTAhUC0GMKHcayBSEQ6AEIOzAE#v=onepage&q=dimittendi%20peccatores&f=false

Oh, that’s a good point. In fact, the Vulgate uses the dative with dimitto in the passage under consideration:
Domine, quotiens peccabit in me frater meus et dimittam ei?
But that brings up a question. What you do with verbs that take the dative, such as dimitto, in a periphrastic construction, which itself takes the dative of personal agency?

You would use a+ablative, which trumps the dative:

Peccatoribus peccata dimittenda sunt a nobis

That makes sense. Thanks.

It’s good to have scruples about these sorts of things but, really, on this occasion they are dimittendi.

dimitto with a personal object regularly means not just to send someone away, but to discharge, acquit, absolve, or release him; its use in forensic contexts makes it obvious why.

Is such a sense not adequate for our present purposes? Previous commentators at any rate seemed ignorant of any inadequacy:

https://books.google.co.uk/books?id=3EwDqGx6_DkC&pg=PA1271&dq="ut+ostenderet+se+omnia+peccata"&hl=en&sa=X&redir_esc=y#v=onepage&q="ut%20ostenderet%20se%20omnia%20peccata"&f=false

https://books.google.co.uk/books?id=uCFiAAAAcAAJ&pg=PA666&dq=quo+beatum+petrum+instruxerat&hl=en&sa=X&redir_esc=y#v=onepage&q=quo%20beatum%20petrum%20instruxerat&f=false

https://books.google.co.uk/books?id=hapbAAAAcAAJ&pg=PA46&dq=id+est+universam+transgressionem&hl=en&sa=X&redir_esc=y#v=onepage&q=id%20est%20universam%20transgressionem&f=false

I did try to insert snippets of the relevant bits instead of links, but the forum software doesn’t provide any obvious means of uploading picture files.

If there is no other way to explain it, we’ll have to accept it as an example of bad Latin (Latinitas ferrea/lutea). The fact that the antiphon was dropped from the liturgy shows that it came eventually to be considered as such.

True. My problem is that I was hoping to resurrect the antiphon as a substitute for Serve nequam, which is a killer to sing. If I do use it, I’ll have to decide between dimittendi, which is by far more frequently found in the manuscripts, and dimittendum, for which there is at least a precedent.

bedwere, far more commentators than the ones I have linked to cite this passage for one reason or another. Of all those I have seen, not one of them makes any suggestion that it is “bad Latin”.

Perhaps you could say what evidence you have that this was the reason it was dropped, i.e. that this use of dimittendi was felt to be “bad Latin”. If you have such evidence then it would certainly bolster your claim that it is, and was felt to be, bad Latin. Without such evidence we can only take your claim that this is the reason it was dropped for what it’s worth.