Can you reuse caverject once mixed?

I have a question. My boyfriend is currently using this....and it's awesome. However, the pharmacy will only give us 2 20mcg vials of powder at a time. We have called around to Pfizer, a few different pharmacists, and the urologist, and apparently this is the smallest vial available at this time.  The first time he used it, he used 7.5mcgs and it was WAY satisfactory, so he wanted to try 5mcgs the next time to see if it will be just as good but won't leave him not able to leave the house for 3 hours after being satisfied more than once, if you catch my drift.  Last night, we did 5mcgs and it was, again, quite sufficient.  We both orgasmed twice and the length of time we got to play was well over 2 hours.  However, even though we only used the 5mcgs, we still had no choice but to mix the entire 20mcg vial.  According to the pharmacist and Pfizer, you are supposed to throw the leftover mix away right away after you use it. So, essentially, we are paying for what could very well be 8 doses, and can only "safely" take 2 doses. So why can't we get the powder in smaller vials, like 5mcgs? I asked pharmacists at both walgreens and cvs and was told they just weren't getting them in any size vial but 20mcgs. So essentially, we want to know if it is safe to go ahead and save the leftover mixture and use it again. Of course both the pharmacy and Pfizer said no, but we used the leftovers last night from the 20mcg mixture we mixed and used the night before, and all seemed fine.  So, do any of you know, is it safe to use the mixed caverject it after it's been mixed 24 hours ago? If so, should it go in the fridge? 

Thanks.  

BTW, he is only 30; I realize that is probably younger than many here, but for whatever reason, this problem hit him at a young age.  He has tried both Viagra and Cialis with no result. This is the only drug that has helped us and we would like to continue to use it for as long as we can.   

Thanks again.