Problem one
The peasant originally had only 21 pesos.
This because, for the first cross, the amount in his pocket multiplied by two, that is, (21×2)=42. He gave 24 pesos to the devil for the deal (42-24)=18
For the second cross, the 18 pesos that was left in his pocket also multiplied by two (18×2)=36, he gave 24 pesos to the devil again, remaining 12 pesos on him.
For the third cross, the 12 pesos left in his pocket also multiplied by two (12×2)=24, but because of the deal, he gave all the 24 pesos to the devil. He finally had nothing left on him again.

Problem two
The inn owner served them a total of 27 potatoes.
The first who woke ate 1/3 of the 27, that is 9, leaving 18 of them for his/her friends.
The second who woke up also ate 1/3 of the remaining 18, that is 6, leaving 12 for his/her friends.
The third person who woke also ate 1/3 of the remaining 12 that is 4, leaving 8 for his/her friends.

In total, they ate 19 potatoes leaving 8

For them to have eaten equally, each person was supposed to eat 9 potatoes.

Thank you @josegonzalito for the contest, and I invite @zology69 and @fadil1 to participate in this contest