Responsa for Shevuot 57:16

Q - (2) B and C claim that the house in Tiberias [in which they were partners with E] was in a very poor condition. B and C, who were in another place, sent money to E, who lived near the house, to repair and renovate it. E, thereupon sent B and C an account of the latter's money he expended in fixing the house, which totaled 40 pounds.
B and C demand that E should pay his share of the expense [in repairing the house] less all rent owed by the former, for the time that they lived in the house. Furthermore, B and C assert that if E states on his word of honor, that he had subsequently given money to the former, then they will waive all their claims to this money, for such was their promise, in the writ of settlement.
E declared on his word of honor, that he owed nothing to B and C, and refused to render any explanation, concerning the matter.
A - (2) The letter which E wrote to B and C, even though it might not have been signed, is an admission to the latter's claim. Therefore, E can no longer aver that he had erred and must give a full explanation regarding the matter.