The demands have brought about a mixed reaction from Sunderland fans, split over whether Dembele can make enough of an impact to give the club a lift.
The alternative would be to spend big on a striker, but the chances are that the club's finances will not extend to doing both.
Dembele's versatility makes him valuable as cover for Christian Eriksen, and it is this ability to feature in a creative role which could see him make a telling impact for Sunderland.
The Black Cats scraped survival last season, and convinced 'firefighter' boss Dick Advocaat to stay on for a further year. Already though there are worries there will be another arduous campaign ahead after an opening day 4-2 loss to Leicester City.
Belgian international Dembele could benefit from a more prominent role at Sunderland, but the price tag Spurs are demanding appears a high one.
Few Sunderland fans doubt his quality, but the prospect of spending the majority of their remaining transfer budget on him is in question.
Here is a look at the split reaction to the rumours via Twitter, with one fan labelling the prospect 'disastrous', and others urging Sunderland to meet Tottenham's demands right away.
Surely we could give Spurs Bridcutt, Graham, Buckley and 14.9 mill for Dembele?? #safc— Ross Sanderson (@RossSanderson25) August 12, 2015
Would prefer to pay the 12 million for Dembele or Townsend over Jordan Rhodes. We have capable strikers just no creativity at all #safc— Kenny Dodd (@KennethRhysDodd) August 12, 2015
£15m for Dembele is too much IMO for what he offers but we are getting desperate now. Rhodes too... would be happy with him but costly.#SAFC— Vauxy1984 (@Vauxy1984) August 12, 2015
Mousa Dembele. Doesn't score, doesn't assist. Zero end product. Would be a disastrous waste of £15m. Why repeat Rodwell mistake? #safc— Parkersafc (@Parkersafc) August 12, 2015
Dembele is a good player. Wouldn't pay £15 million for him though #safc— Philip Marsden (@Parsden) August 12, 2015
So people don't want Rhodes because he's too expensive, but would happily stump up £15m for Dembele. Right... #SAFC— Michael Potts (@potts93) August 12, 2015
So Spurs want 15mil for Dembele, do it! Break the bank, he's on a different level #SAFC— Joe Robinson (@Joe94R) August 12, 2015
£15m for Dembele? Snap their hands off Ellis. Just what we need in our team, but of class, strong as a bull. Get him bought #safc— david laing (@davidlaing93) August 12, 2015