All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad is His slave and Messenger.
It is impermissible for you to tell your future husband a higher price for the things he wants to buy for you to deceive him and take his money against his will for this is considered deception, which is forbidden in Islam. The Prophet, sallallaahu ‘alayhi wa sallam, said: “Whoever cheats us is not one of us.” [Muslim]
Furthermore, this act is regarded as consuming another person’s money unlawfully; Allaah The Almighty Says (what means): {And do not consume one another's wealth unjustly or send it [in bribery] to the rulers in order that [they might aid] you [to] consume a portion of the wealth of the people in sin, while you know [it is unlawful].} [Quran 2:188]
Moreover, this may ruin your future marital relationship with this man if he finds out what you have done.
Allaah Knows best.