The World Federation One Stop Fiqh

Ritual 54

It is possible for a donor to withdraw his gift before or after the beneficiary assumes ihram. However, if the donor withdraws it after the beneficiary enters into the state of ihram, it is ,most evidently, obligatory on the beneficiary to complete the pilgrimage, only without undue hardship; that is, even though he actually did not have the means. The donor should reimburse him with the expenses incurred.

And if the donor withdraws the gift while the beneficiary is en route to pilgrimage, he is responsible for bearing the expenses of the return journey.