Qitzur Shulchan Arukh – 180:10-11
Someone who sells some thing to his friend on credit, it is as though he lent the money, and [the implied loan] retires. However, a shopkeeper who sells to others on credit, and it isn’t his way to collect until it accumulates to some amount, [the tab] does not retire. However, if he stands over it like it’s a loan, which means he calculates the whole thing and writes in his notebook the running total, then it is like a loan and does get retired.
A workman’s pay is not retired. However, if he stands over him like it’s a loan, it does retire.
One concept of shemitas kesafim, the retirement of loans on the shemittah year, is to put a cap on how long someone can be hounded by his creditor. Therefore, the the lender doesn’t have the right to collect the loan yet, shemittah doesn’t terminate the lown. Similarly, a seller who uses credit but only mentions the tab when it gets to a certain size isn’t subject to shemittah, since a sale on credit isn’t fully a loan, and the ills shemittah is there to eliminate aren’t involved. Similarly most employees who wait for payday. However, if storeowner or worker makes a big deal about the credit each time an amount is added to it, then it is sufficiently like a loan in the very way that matters to qualify.