I believe you are interpreting Reg CC incorrectly. You have admitted to not placing a hold on the $300 item, yet want to also count $100 of the $300 towards the $2,000. My interpretation would be that you have actually placed a hold for $100 of the $300 using your method.
Check your state banking laws as well. In some states, the hold is relative to the entire deposit, in which case you would be okay. In others, is is on a check by check basis, regardless of the total deposited. To play it safe, why not give the customer $100 of each and save yourself a headache?
Also, what do your deposit guidelines state? (Don't you just hate those auditors?)
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