Money talks, OP arrangement was a down payment with balance in 30 days. There was NO mention of having complete payment at time of meeting. My rifle, my choice of buyer. Discussed via text and buyer called offering full payment after stating in initial contact difficulty coming up with funds. Buyers talking about money problems do little to inspire confidence.
Then why agree to his proposal and tell him he has first dibs....
Backing out on your word does little to inspire confidence - in honesty or character.