Declining an offer to present a poster instead of a paper I have submitted a paper to a conference which defined several submission categories: papers, posters, etc. I submitted my abstract in the paper category, but was asked to present a poster instead.

I would do it if it was practical in my situation. But the conference is in another country, and the travel expenses are somewhat consequent. I'm not sure my research lab would accept to cover the expenses, because we're already on a tight budget. Even so, I don't want to be stuck later on if my paper gets accepted somewhere else and I can't go there because I already spent too much of the lab's money on this poster. I'm not even certain I could present the exact same research in another setting without it being perceived as a duplicate.

I hence want to decline the offer (hoping this isn't a mistake!). Do the arguments I gave above sound reasonable, from the point of view of a conference committee? Is there something I could add or remove to make this better?
I don't think  you actually need a reason to decline. Saving the work for submission elsewhere is perfectly reasonable in any case. All you need to reply is "No, thank you". 

Posters are good for preliminary work and for students wanting some exposure and an opportunity to meet other researchers, of course. 

But the work is  yours and you don't need to explain why you prefer to withhold it at this time.