Traveling back to invest in time travel

[C10]

Trying to find this wonderful story in which someone goes back about 700 years, invests one gold piece, returns at 100-year intervals to guide the fund's growth. Then cashes it all out in the 1960s (?) to buy enough energy to run the time machine.

Last couple of lines are especially brilliant. I decided recently that it's actually a true story, about a certain family of wine and banking note, and am wondering what their final act will be.

Key word searches didn't find it for me - would appreciate help identifying title and author.

Thanks-

[LianeWayfarer]

"Compounded Interest" by Mack Reynolds. The only two collections I know it appeared in was Judith Merril's YR'S BEST SF 2 (dell, 1957) and as the title story in NESFA Press' COMPOUNDED INTEREST, the book issued in honor of Mack Reynolds' Boskone GoH.

[C10]

That is clearly the desired story, and I'm very much obliged for your courtesy and knowledge.