Ari operates a business as a designer of internet web pages for a variety of business clients. Unfortunately he has had some difficulty in recovering his full fees from a number of clients as follows:

(a) Bi, a newly qualified accountant, told Ari that although she could only raise the cash to pay half of the outstanding fees she would, as an alternative to paying the other half, do all of Aris accountancy work for the coming year. Ari reluctantly agreed to this proposal.

(b) Cas, a self-employed musician, told Ari that she could not pay any of the money she owed him. However, her father offered to pay Ari, but could only manage half of the total amount owed. Once again Ari reluctantly agreed to accept the fathers payment of the reduced sum

. (c) Dex, a self-employed car mechanic, without contacting Ari, simply sent him a cheque for half of his fees stating that he, Dex, could not pay any more and that the cheque was in full settlement of his outstanding debt. Ari himself is now in financial difficulty and needs additional cash to maintain his business operation

. Required: Advise Ari whether, in the context of contract law, he can recover any of the outstanding money from Bi, Cas and Dex.