Someone travels to work out of town with his own car. He received a credit card from his employer to pay for the gas for the trip to and from the workplace and his home. The employer does not mind if he makes short local trips before or after work for his own purposes, although he is using gas which was paid for by he employer. when he travels on longer personal trips he is supposed to pay for his own gas. Sometimes, instead of filling the car with gas before a personal trip he will instead make the trip using gas already in the car which he had previously paid for with the employer's credit card, and as reimbursement to the employer for the gas used for his personal use, he will fill the car with gas before he goes to work the next time using his own credit card. With this he then travels to work on the gas which he paid for himself, and thus he has repaid his debt.
The question which arose is as follows: Since sometimes the trip which he traveled using the employer's gas may be shorter than the trip to and from work which he paid for with his personal credit card, it would turn out that he repaid the employer more than he "borrowed". Would there be a ribbis problem with such a scenario?
We must also consider the point that the price of gas may have risen by the time he returns it. If it indeed did than he would be returning more value than he took, even if he returned the same quantity of gas as he used. MORESO even if the price DID NOT go up, we may be involved in a question of "seah besah, which forbids borrowing any expendable item with intent to return another of the same item. The reason for the prohibition is because the price of the may rise by the time he returns it. Nonetheless, the prohibition of borrowing Seah Besah is EVEN IF THE PRICE DOES NOT GO UP. The chachamim made an issur because it MIGHT go up. So too here to we might have that problem even if he returns exactly the same amount as he took, and the 2 trips were of exactly the same length, and the price of gas did not rise, it still may be forbidden because of Seah Besah as explained. One of the exceptions to the rule of "seah besah" is called "yesh LO" This means that if at the time of the borrowing, the borrower already has in his possession any amount of the item being borrowed he may borrow the item and we are not concerned about a future price rise Thus, if the car owner had any of his own gasoline in the car at the time which he used the employers gas for himself, there would be no concern of 'seah besah'