1000iu / 2.5mL means 1.25mL is 500iu, not .25mL - so I'm assuming you mean this is a 5000iu ampule or vial?
Maybe you waste a little but that wont kill you. Next shot just use your desired amount. You can wait a week if you want, not sure its active for more than 3-5 days though.
Didn't you get insulin pins? I notice it leaks out of even 25g pins, I think mine are 30g .5mL slin pins, they work great.