Influence

For any commercial transaction, the spoken word can never be relied upon due to a number of reasons
1. The person may say he or she never made the statement at a later date.
2. There are no records kept for reference
3. The listener may not have heard what was said properly due to a number of reasons like background noise, accent of the speaker, speaking too fast.
So any discussion should be documented in writing, with all concerned signing the document to ensure that there is no misunderstanding.