“ The payment made was for that cargo as an 'outside purchase,' and not a payment under the contract for Wallsend coal. Neither is the agreement to take that cargo, and the payment made for it, to be regarded as a waiver of any difference that might exist between the quality and fuel value of the coal so purchased and the coal which should have been supplied. ”

“ That it does not constitute a waiver results from the agreement that it should be considered 'an outside purchase,' and the express notice before delivery that the contractors would be charged with that difference. ”