I have always thought the meaning of this exchange was clear: <PERSON> would have to sell his speeder to raise enough money for the down-payment on the trip to Alderaan.
<PERSON> had no obvious assets. If he had a treasure chest full of gems or something, this should have been shown.
<PERSON> was a teen-age farm boy, not rich, so he didn't have any money on hand. Possibly <PERSON>'s Aunt and Uncle had a bank account or something, but it would be impractical for <PERSON> to try to get money from it, with the Empire looking for him and the droids. And I doubt anything valuable was left in the blasted farm.
<PERSON> cleverly offered a deal to <PERSON>: a modest sum now, and a generous sum on arrival at Alderaan. (Clearly he was counting on the royal family of Alderaan helping him out.)
Later, when <PERSON> has sold his speeder, he complains that he didn't get very much for it, and <PERSON> says "it's enough". I have always assumed he meant "it's enough to cover the down-payment I promised."
EDIT: The question has been edited with this new text:
<PERSON> says "That's okay", alleviating a concern. He appears to be saying, "that's fine, it doesn't matter that I have to lose my speeder, and the reason I'm okay with that is because I will never return to Tatooine."
Why is this a reason to be okay with losing the speeder?
I don't read too much into this. 865
It would be expensive to ship the speeder as cargo, and the speeder might not be useful on other planets (e.g. on Coruscant, you need something that can actually fly, not just hover a small distance above the ground). The only possible question would be whether <PERSON> has a sentimental attachment to the speeder. (For example, I had a car that I drove for 20 years, and I had some sentimental attachment to it, much more than the car I am driving now.) We can take this comment as <PERSON> reassuring <PERSON> that he has no sentimental attachment to the speeder, and instead of saying outright "I am not attached to this speeder" <PERSON> comments that he is never going to return. This clearly implies that there is no reason to try to store the speeder for later recovery so they might as well sell it.
Also, <PERSON> didn't love living on Tatooine; he yearned to go elsewhere. And his Aunt and Uncle had just been killed on Tatooine, so perhaps now he not only yearns to go, he also hates to stay. I seem to recall him saying "There's nothing for me here, now."
If you like, you can also infer that maybe the speeder reminds <PERSON> of Tatooine, and he would just as soon be rid of it for that reason. 865