Thats the point. He wont/cant clear it. Ayase is portrayed as utterly useless & clueless. One has to wonder how he survived as long as he did b4 ending up w/ kanou.
Your math is a little off. 2 billion/500,000 = 40,000 (2*2*1,000). So it would take him 1,095 years to pay off the debt with sex once a day, or ~46 years with sex once per hour. So still impossible but slightly less ridiculous?
wait i calculed, if he's debt is 2 billion and he earns 500 thousand each time they have sex... Akane must sleep with him 4 MILLION TIMES!!!!
if akane sleeps with kanou once per day, he will clear his debt after 10 951 years, if akane sleeps with kanou every hour, each day ( so 24 times per day), he will clear his debt after 456 years....
O.o WTF??!!! ╮( ̄▽ ̄)╭