In Article 21 of Commission Delegated Regulation (EU) 2015/61 the reference is only to collateral to be received and not to collateral posted.
According to Articles 21, 30(4) and 32(5) of Delegated Regulation (EU) 2015/61, liquidity outflows and inflows expected over a 30 calendar day period for the contracts listed in Annex II of Regulation (EU) 575/2013 and from credit derivatives shall be calculated on a net basis by the counterparty subject to the existence of bilateral netting agreements. Net basis shall be considered to be net of collateral to be received/ posted in the next 30 calendar days. The collateral to be received will only be considered in that net amount if it qualifies as liquid asset, under Title II of the Delegated Regulation (EU) 2015/61, and can be reused by the credit institution when received, as provided in Article 21(3). provided that it qualifies as a liquid asset.
With regard to Article 32(5) of Delegated Regulation (EU) 2015/61, where contracts listed in Annex II of Regulation 575/2013 (CRR) or credit derivatives are collateralised by liquid assets under Title II that have to be returned to the counterparty within 30 days, cash inflows arising from these contracts should be calculated net of any corresponding collateral (captured in row 240 of C 74.00).
Pursuant to Article 30(4) of Delegated Regulation (EU) 2015/61, where contracts listed in Annex II of Regulation 575/2013 (CRR) or credit derivatives are collateralised by liquid assets under Title II that have to be returned to the institution within 30 days, cash outflows arising from these contracts should be calculated net of any corresponding collateral (captured in row 340 of C 73.00).
This is consistent with the principle that banks should not double-count liquidity inflows or outflows.
As regards the first part of the question, the treatment of cash collateral is specified in EBA answer to Q&A 2014_1089.In relation to the second part of the question, according to Article 21 of Regulation (EU) 2015/61 the treatment of cash collateral shall apply to collateral that has already been received or posted by the institution up to close of business of the reporting date.
Update 26.03.2021: This Q&A has been updated in the light of the changes introduced to Commission Delegated Regulation (EU) No 2015/61.