In the classic case of hafkaas halvaah the borrower tells the children of the malveh who passed away that the chov was paid up when actually it wasn’t. Isn’t there an issue of midvar sheker?
If the child of the lender asks for the money back because he knows that the money was owed, it has to be returned, and yes we are not allowed to lie about it.
Have a Kasiva V’chasima Tova
CH”M 281-1, SM”A ibid 6. CH”M 228-6, Pischei Choshen (Geneiva) 1 ftnt. 3.