A poor person cannot settle for a lesser amount of zakat before receiving it, nor can he accept something more expensive than the value of the zakat. Furthermore, the giver of zakat cannot give it to someone entitled to receive it on condition that he must return it to him. However, there is no problem if the entitled person consents to return it to him after receiving it.
For example, if someone who owes a lot of zakat but has become poor and cannot give it, and he has repented, and a poor person consents to take his zakat from him and gift it back to him, there is no problem.