Should I refuse my wife's financial help to buy a house, in order to please my mother who rejects this money?

The son is obligated to obey his parents in matters that benefit them and do not harm him. However, there is no obedience to them in sin or in matters that do not benefit them. Your mother has no interest in you not acquiring money, and you may acquire it without her knowledge. Enriching and financially empowering one's wife is a legitimate objective in marriage. Allah has permitted the husband to take from his wife's wealth whatever she willingly gives. A Muslim's wealth is not lawful except with their willing consent. If you request money from your wife or fear her indebtedness, it is better to put the house in her name and safeguard her money therein. This might also be easier for your mother.