YES

(format LCTRS :logic QF_LIA)
(fun return 1 :sort (Int Unit))
(fun sumfrom 2 :sort (Int Int Unit))
(fun tmp 2 :sort (Int Unit Unit))

(rule (tmp m_0 (return n_1)) (return (+ m_0 n_1)) :vars ((m_0 Int) (n_1 Int)))
(rule (sumfrom m_2 n_3) (tmp n_3 (sumfrom m_2 (- n_3 1))) :guard (<= m_2 n_3) :vars ((m_2 Int) (n_3 Int)))
(rule (sumfrom m_4 n_5) (return 0) :guard (> m_4 n_5) :vars ((m_4 Int) (n_5 Int)))

Confluent by Almost Parallel Closedness with proof:
no critical pairs

Elapsed Time:  19.36 ms