To-day I sent an envelope to Cologne to obtain an autograph from Ms. Lena Meyer-Landrut (the recent Eurovision winner). To do so I put an enveloped (with my address on it) inside the enveloped to Germany so the people there could send the autograph inside the envelope to here. When I asked the clerk what stamp to put on the inside envelope he informed me that there was once international stamps that were sold but no longer and for me to put a dollar with the inside envelope in the outer enveloped for them to get a stamp (when I asked how a US dollar could be workable in German Federal Republic he told me that they were exchangable). So is this actually workable?
There weren’t international stamps, but rather International Reply Coupons. They could be exchanged at the foreign post office for local stamps good for sending one international letter. They were used quite widely, but, like traveler’s cheques, have been displaced by electronic payment systems like Paypal and credit/debit card networks.
Edit: Link from Canada Post.
The usual practice is to enclose anInternational Reply Coupon which the recipient can exchange for postage in their home country. More info from the USPS HERE.
Wow, a triple! 
IRCs (and International Business Reply Envelopes, slightly cheaper if you’re just getting a piece of paper back) are still in use, but apparently some postal employees are either unaware of these services or unwilling to sell them to you.
And to add to your question, the recipient should be able to exchange the US dollar for however many euros it’s worth, then go to buy a stamp. But banks often charge hefty processing fees for changing currency; my bank charged $5, I think, and the booths at Toronto airport charge 7.5% (!).
According to this site, it costs €1.70 to send a letter from Germany to places outside Europe. That’s $2.08 US.
I sent only a dollar as suggested by the clerk.
Your dollar’s wasted, I’m afraid. The service charge for exchanging foreign banknotes will certainly be at least (the euro equivalent of) one dollar.
Sell them to you? Try getting them to redeem one.
IRC, as mentioned above.
But if Ms. Meyer-Landrut’s management are any use they’ll accept your dollar in lieu of a SAE - there’ll be a number of other people who send banknotes, and any bank branch will accept them for exchange or deposit.
As a amateur radio operator, I send out lots of SASEs in order to receive QSL cards back for contacts. The standard is two green stamps ( dollar bills ) to any country other than Canada or Mexico. I haven’t bothered with IRCs in years.