conditional offerings to gods.
Like, If I get so and so thing then I will do such and such thing for you.
First of all it is in bad taste to presume that the gods are greedy after what you are likely to offer on fulfillment of the wish and put them to test on such assumption.
Secondly, it is against the sastras.
Meemamsa puts Vigrahaha (invocation through mantras), Havisveekaraha( acceptance of the offering, tadbhojanam (its enjoyment), truptihi( satisfaction with the offering) as prerequsites for obtaining the devata's prasdaha (blessing)