Responsa על בבא בתרא 335:12

I ruled that Shimon must give him a copy, and my colleagues agreed. They later showed me a responsum of Rosh (Responsa Rosh 68:24) stating that, indeed, a copy of all documents must be submitted for scrutiny. I read this carefully, and it implies that [the litigant] does not concede that there is a contract, but says that he can ascertain from the writing that it is forged or has already been paid. However, if one concedes that there is a contract but wants to undermine it, it seems that he should not be given a copy. For it is stated in the chapter “[Get] Pashut” (Bava Batra 168a): “It is unpleasant for me for my claims to be in your hands.” And Rosh explained this that specifically in the case of a contract, a copy must be submitted, for if it is forged, then all is well, and if it is not, he will not be able to discredit it as a forgery and ruin it. So what harm is there? On the contrary, justice will come to light. Statements of claim, on the other hand, should not be given to the other litigant.