YES

(format LCTRS :logic QF_LIA)
(fun return 1 :sort (Int Unit))
(fun sum1 1 :sort (Int Unit))
(fun u 2 :sort (Int Int Unit))
(fun v 2 :sort (Int Int Unit))

(rule (v n_0 s_1) (return s_1) :guard (not (>= n_0 0)) :vars ((n_0 Int) (s_1 Int)))
(rule (v n_2 s_3) (v (- n_2 1) (+ s_3 n_2)) :guard (>= n_2 0) :vars ((n_2 Int) (s_3 Int)))
(rule (u n_4 s_5) (v n_4 s_5) :guard (not (< n_4 0)) :vars ((n_4 Int) (s_5 Int)))
(rule (u n_6 s_7) (return 0) :guard (< n_6 0) :vars ((n_6 Int) (s_7 Int)))
(rule (sum1 n_8) (u n_8 0) :vars ((n_8 Int)))

Confluent by Weak Orthogonality with proof:
no critical pairs

Elapsed Time:  10.10 ms