"You shall observe shemittah. And this is the word of the shemitah: Every creditor shall release his claim" - As long as you have shemittah (of fields) you must observe shemittah (of releasing claims).
"seven years": I might think (that a period of) seven years (is allotted) for each borrowing (before the loan is "released"); we, therefore, reason: Seven years are required until shemitah (non-working) of the land, and seven years are required until shemitah (release) of loans. Just as in the first instance, the shemitah is universal (i.e., at the same time for all lands), so, in the second, the shemitah is universal (i.e., at the same time for all loans).