Is it permissible to lie or fabricate an excuse to sell a share in an inherited building without displeasing the mother or the partner?

The fundamental principle is that a Muslim should strive to obey his mother unless it causes him hardship. If it does cause him hardship, then obedience is not obligatory. There is no objection to selling your share without her consent. If you fear her anger, you can use ambiguity and veiled hints to avoid lying.