The operating company should provide a letter naming the listed profession, the duties, the qualifications required and the expected period. The payroll organisation can confirm compensation, benefits administration and its role. Together they should describe one consistent arrangement rather than two competing accounts.
Reconcile the documents before relying on either
Inconsistencies surface easily here: a title on the payroll record that differs from the hiring manager's description, or a start date that reflects onboarding rather than the intended commencement of work. Compare the documents line by line and have the discrepancies corrected at source. A document amended informally after the fact carries less weight than one issued correctly the first time.
Dates are the part of these letters least often checked. A duty description written for a role beginning next quarter should say when the position was approved, when the work is expected to start and how long it is expected to run; a document carrying only a signature date leaves the period to be inferred. Where the payroll organisation's confirmation predates the operating company's letter by several months, expect the older one to describe an arrangement that has since moved.
Ask for reissue rather than annotation. One record settles more than the letters do: the service agreement between the two companies, which shows on its face the term, the renewal mechanism and which party controls the professional's assignment.