Chullin 131a - Matanos and the king's collection

I'm having a hard time with the gemara about Matanos and אין לו תובעין, just trying to get the basic ideas. One place:
The gemara says that if the king collects taxes from a person, taking matanos in payment, the person has to give the matanos' value afterwards. It takes that as proof that מתנות יש לו תובעין. I wasn't sure why.
It's clear that it isn't because the person has a חיוב שמירה, like a שואל, because the second case there is when the matanos were taking by robbers, and there is no obligation to repay.

Here's a tentative explanation: Reuven owes money to Shimon. Shimon comes to collect, and finds something belonging to Levi in Reuven's house, and just takes that. Reuven is now off the hook from Shimon. Levi has no way of collecting from Levi for whichever reason.
Now can Levi go to Reuven and say, my money was used to pay your debt - now pay me back for my loss?
If so, that's the case in the gemara. יש לו תובעין means, the matanos already actually belong to the cohen. So the king used the cohen's money to collect his debt - now the owner has to pay the cohen back.
Whereas, if אין לו תובעין, the matanos still belong to the owner, only he has a mitzvah to give them to the cohanim. Once they are gone, there is no longer any reason to give them.

Does this sound right?

The next gemara is also interesting: if a person sells the matanos to someone else, that buyer has to give them to the cohen, and then may or may not be able to get his money back, depending on how the sale was done. The gemara takes that as evidence that יש לו תובעין, since the cohen can collect his money even from the buyer - it's his money, after all!
But then the gemara answers, Not a proof. It's different if the matanos are still existing (unlike earlier cases when they were eaten or destroyed).
Now how does that work? They don't belong to the cohen yet, there's just a mitzvah to give them. So what are we concluding, that the mitzvah is on the object of the matanos, not on the person, and it doesn't matter who has them?
Whereas (in the earlier gemara), if they were destroyed, and משתרשי ליה, the owner profited, he still personally has a mitzvah to give their value? How does that work?