sa is substituted for saha in an avyayībhāva samāsa when the second member is not a word denoting time.,
Thus सचक्रंधेहि, मधुरं प्राजः, but सहपूर्वाह्णम् the second member being a time denoting word (II. I. 5).,
अव्ययीभावे चाकाले
Adhyāya 6 · Pāda 3 · Rule 81