Veganmartin,
To answer the last 2 questions you asked.
1] "Proteinuria" is another indicator of kidney function. It generally is either any protein in the UA [urinalysis] by dipstick, or an above normal amount in your 24 hour urine collection for creatinine clearance [if that test was ordered]. Proteinuria indicates possible kidney disease. I assume you do not have that problem, otherwise the transplant center would probably have mentioned it. You can also check your lab results.
2] "Renal scan" is a general term that can mean several different tests. (You can ask your center what test they mean.) I assume your center means an Iothalamate test (or similar test) that is an accurate measure of GFR. Yes, that scan is more accurate than a 24 hour creatinine clearance, that in turn is more accurate than the "eGFR" [GFR estimated from a blood test] when the 24 hour test is done under the best conditions.
Why should a donor's GFR be above a minimum number? Two reasons.
A] The future health of the donor after donation. The kidney remaining in the donor compensates somewhat for the loss of its partner kidney by increasing its own GFR [= Glomerular Filtration Rate]. But it can do so only to a certain point, usually reaching 65% to 75% in most donors of what their GFR had been with 2 kidneys. Although some centers accept donors with a GFR as low as 80, some require that the GFR be higher, especially if the donor is young, as you are. A donor with a pre-donation GFR of 81 would end up post-donation GFR of 53-61 -- the lower part of that range being lower than most centers would accept as "minimizing harm" to a young donor.
B] The health of the recipient. The transplanted kidney in the recipient could compensate only as high as the remaining kidney in the donor -- and often only to a yet lower level for several reasons. First, if the donor's two kidneys have unequal GFR between them, many centers take the kidney with the lower function, so that the donor's health is harmed the least. Second, the medicines used to prevent rejection impair kidney function to varying degrees. There are other reasons, too. Bottom line: most centers are careful to put the donor at risk only when the likely outcome is low enough harm to the donor and high enough benefit to the recipient.
Both Sherri's and Janice's replies indicate that they were concerned about your GFR. I am, too. I agree with both as well: discuss your results with your PMD and also with an independent nephrologist she or he recommends. Because eGFRs and even 24 hour creatinine clearances are sometime inaccurate -- either higher or lower than "true GFR" -- it is possible the results of an Iothalamate scan (or similar test) will show that your GFR is high enough to do what you want to do, i.e., donate.

All the best to you and your-brother-in-law.

B