In a couple areas of Bava Kamma and Bava Metziah I've seen the term נושא שכר listed with a shomer chinam, a socher, and a sho'el. Is the noseh sakhar a synonym for a shomer sachar, or are these distinct concepts?
Bava Metzia Perek 7 Mishnah 8 on Sefaria.org שׁוֹמְרֵי פֵרוֹת אוֹכְלִין מֵהִלְכוֹת מְדִינָה, אֲבָל לֹא מִן הַתּוֹרָה. אַרְבָּעָה שׁוֹמְרִין הֵן. שׁוֹמֵר חִנָּם, וְהַשּׁוֹאֵל, נוֹשֵׂא שָׂכָר, וְהַשּׂוֹכֵר. שׁוֹמֵר חִנָּם נִשְׁבָּע עַל הַכֹּל, וְהַשּׁוֹאֵל מְשַׁלֵּם אֶת הַכֹּל, וְנוֹשֵׂא שָׂכָר וְהַשּׂוֹכֵר נִשְׁבָּעִים עַל הַשְּׁבוּרָה וְעַל הַשְּׁבוּיָה וְעַל הַמֵּתָה, וּמְשַׁלְּמִין אֶת הָאֲבֵדָה וְאֶת הַגְּנֵבָה: Those who guard produce may eat according to civic law but not according to the Torah. There are four types of guardians: an unpaid guardian, a borrower, one who collects a wage, and a renter. An unpaid guardian may swear in every case; and the borrower should pay restitution in every case; and one who collects a wage or a renter must swear concerning an injured [animal], concerning one that has been carried off, or concerning a dead [animal], and must pay restitution for loss or theft.
The Peirush Hamishnah of the Rambam gives the biblical origin of the laws for each of these guardians, and uses the term Shomer Sachar to refer to Nosse Sachar in the Mishna.
The two terms are synonymous.