The entry in question reads:
“τεμπεσίρι (noun): chalk. By extension, the board on which customers’ debts were written in chalk.”
What source tells us that “τεμπεσίρι” was also used (by extension) to refer to the board, etc.?
Personally, I believe that the entry should be amended as follows:
Τεμπεσίρι: 1. Chalk; 2. (figuratively) credit