Can he send the poor person to collect the debt owed to him and regard that as zakaah?

Waiving debt and regarding it as zakaah is not valid and
does not take the place of zakaah, because one of the conditions of zakaah
being valid is that it be handed over to the poor person and that the
latter should take possession of it. Cancelling a debt is waiving it, but
does not involve the person taking possession of anything. Allah, may He
be exalted, says (interpretation of the meaning): “If you disclose your
Sadaqaat (almsgiving), it is well; but if you conceal them and give them
to the poor, that is better for you” [al-Baqarah 2:271]. 

Shaykh Ibn ‘Uthaymeen (may Allah have mercy on him) said in
his Tafseer: One of the things that we learn from this verse is that
sadaqah is not regarded as such until it reaches the poor person, because
Allah says “and give them to the poor”. See also the answer to
question no. 13901 and
119113. 

But the way asked about here is not waiving a debt; rather it
is delegating the poor person to go and take his right from So and so, and
regarding that as zakaah. There is nothing wrong with this and it is
acceptable. 

But if the poor person does not take possession of it because
a debtor is taking too long to pay or is in difficulty, then you still have
to do give zakaah, because in that case your duty has not been discharged. 

Al-Sarkhas (may Allah have mercy on him) said: If he gives
what is owed to him in charity to another poor person and tells (the poor
person) to take it from him with the intention that it is his zakaah, it is
acceptable as such, because that poor person is acting on his behalf with
regard to taking it; so it is as if he took it himself then gave it to him
in charity with the intention that it be zakaah. End quote from
al-Mabsoot, 3/36 

And Allah knows best.