Responsa על בבא בתרא 227:2

Q. A formally released his mother, L, from the obligation of taking the required oath regarding her ketubah, and gave her the right to will the property to whomever she might please, to sons and to daughters, to give more to some and less to others, on condition that she transfer to A a particular piece of real property forthwith. Immediately after the formal release, however, A said to his mother that since this real property was mortgaged, her release was not to become effective till she redeem it for him. L promised to redeem it. Now, however, she refuses to do so.
A. Unless L redeem the above property, her agreement with A is void, for A's last stipulation proves that he had originally agreed to release L on condition only that she transfer the property to him unencumbered by mortgages. Moreover, a stipulation made immediately after an agreement is concluded, but while the matter is still being discussed, becomes part of the agreement (B. B. 114a).
SOURCES: Cr. 96.