Can a mortgagor, if he knows that an underlet estate is underlet, charge a mortgagee for rent that may be higher than the underlet amount?

Also Hughes v. Williams, 12 Ves. 493 (8 R.R. 364): Another circumstance that weighs with me, is, that the mortgagor, if he knows the estate is underlet, ought to give notice to the mortgagee, and to afford his advice and aid, for the purpose of making the estate as productive as possible * * Can the mortgagor lie by, not giving notice that a greater rent may be made, and come afterwards by way of penal inquiry, to charge the mortgagee with the effect of his own negligence?

Also the Ontario case of Coldwell v. Hall, 9 Gr. 110, at p. 114. Vice-Chancellor Blake concluded his judgment by saying: The Master seems simply to have inquired after one matter, and that was, could the mortgagee have possibly procured a higher rent than he has done. He found this against the defendant, and then implied as a result that there must have been wilful default or neglect. The authorities show that, in addition to this question, must be considered whether the small or lost rent arose from the gross default, mismanagement, or fraud, of the person sought to be charged; and, if not, an account of actual receipts alone can be had in favor of the mortgagor.