Luxembourg stamp of a Nazi bill

I have the opportunity to purchase this bill for $20, which seems to be a Luxembourg stamp on a Nazi 5 reichsmark. My question is, worth it or not? I cannot find this bill anywhere online (the stamped version), so I can't get a base value of what it may be worth. I only have a picture of 1 side.

Probably stamped like that to commemorate the liberation of Luxembourg in 1944. I'd say it's worth what someone would pay for it, but then again I wouldn't pay $20 for it myself.

Verweis : "CassTaylor"​Probably stamped like that to commemorate the liberation of Luxembourg in 1944. I'd say it's worth what someone would pay for it, but then again I wouldn't pay $20 for it myself.

​Thank you