Excellent question! The commentaries have a number of approaches to resolve this apparent contradiction. Here are a few that I came across:

1) On 52b there is a problem because the relatives of Rebbi Yochanan went to arrange the fixed-price deal with the doctor without the consent of the widow; doing so against her will makes it a problem for Rebbi Yochanan to provide them this advice. On 54b, however, there is no problem, since no one is forcing onto the wife the arrangement to the designated land; the wife can decide of her own volition whether to accept the agreement or not (Tosfos 54b, DH d'Neyached).

2) Besides the above consideration, one can understand that in the case on 54b the relatives are only trying to prevent the wife from overindulging, but not from partaking of her fair share of sustenance. Therefore, it would not be wrong for Rebbi Yochanan to provide them this advice (Ritva 54b, DH d'Neyached).

3) Rebbi Yochanan actually regretted having given the advice in both stories. The story on 54b happened first, and the story on 52b happened second. When Rebbi Yochanan expressed his remorse it was regarding his actions in both incidents (Rashi cited in Shitah Mekubetzes).

I hope this helps!

Warmest regards,

Yishai Rasowsky