What should the reference for this rule of law?

On the present state of the law on this subject can be expressed in the following proposition: (i) if A has entered into a contract with B to do work for, or to supply goods or services to, B in return for payment by B; and (ii) at some stage before A has completely performed his obligations under the contract B has reason to doubt whether A will; or will be able to, complete his side of the bargain; and (iii) B thereupon promises A an additional payment in return for A's promise to perform his contractual obligations on time; and (iv) as a result of giving his promise, B obtains in practice a benet, or obviates a disbenefit; and (v) B's promise is not given as a result of economic duress or fraud on the part of A; then (vi) the benet to B is capable of being consideration for B's promise, so that the promise will be legally binding