Having regard to Article 11(C)(1) and the second sentence of Article 20(1)(b) of Directive 77/388/EEC in relation to the downward adjustment of the taxable amount and the adjustment of the VAT charged on taxable transactions in cases where the consideration agreed by the parties remains totally or partially unpaid, is it compatible with the principles of proportionality and effectiveness guaranteed by the TFEU, and the principle of neutrality that governs the application of VAT, to impose limits that make it impossible or excessively costly — in terms of time too, in connection with the unforeseeable duration of an insolvency procedure — for the taxable person to recover the tax on the consideration which remains totally or partially unpaid?