Unable to pick up Office to Office (OTO) Disbursement posting in bill

It sounds like when posting the OTO the 'Tick if For Bills' option might have been selected, if so this will hit your Debtors control and not be available to be picked up in a bill.
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You can double-check this by double-clicking on the posting and looking at the nominals that have been hit. If your Debtors Control forms part of the double entry, then the 'Tick if For' Bills option was selected. To fix this perform an OTO in the opposite direction and Tick 'Tick if For Bills'.
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Then make a new OTO and keep and keep the 'Tick if For Bills' unticked, this will then hit your Disbs Unbilled control, and allow you to pick up the Disbursement in a bill.