From Latin summa. The word was probably originally inherited and popular, as evidenced by its appearance (also in an older form somă) among the earliest Romanian texts from the 16th century, and the popular nature of some of its derivatives and related terms. However, the modern version of the term seems to have been borrowed through the intermediate of another language, possibly Polish suma, and replaced the original form.  See also Aromanian sumã, which was inherited.
sumă f (plural sume)