On DPS all bank items needed to be posted in order to Post a bank rec.
For this scenario there are 2 possible workarounds:
Postings required
Post a corresponding Client to Office transfer for £2k to a Misc/Suspence matter. In the detials be transparent about the error which occurred and reference the matter number on which the error was triggered from. This can then be matched off the bank rec and enable the Post button to appear.
On the next day of the new month, post an Office to Client transfer back to correct. This entry will match off of the bank rec.
No postings required
On your bank rec screen, Enter your Bank Statement Balance as £2k less, to allow the Post to appear, and enables you to get your rec posted, then mark on the bank rec printout what happened.
When your new period's bank rec is done you can just ignore the contra in/out £2k and reenter the true bank statement balance for the next month. This would get you around the issue of having to post fictitious amounts to Misc ledgers,
In both examples, it is recommended that you log this as a breach with your firm's COFA.