Charge transfer is a fundamental process that lies at the heart of a wide variety of fields, including bioscience, physical chemistry, optoelectronics, and 2D semiconductor devices. The control of charge transfer dynamics requires localization of an initial electronic wave packet, which can be achieved by element-specific core excitations driven by X-ray radiation sources. Resonant Auger scattering provides a unique tool to probe ultrafast charge transfer with characteristic times in the femtosecond to attosecond range. To date, the main X-ray-based method to measure the charge-transfer time is the core-hole clock technique, where the very short lifetime of the core-hole state is used as an internal reference time. Here we demonstrate that this method is only applicable in a regime where direct population of the charge-transfer band by X-ray core excitation can be neglected. To overcome this limitation we derive a new method to study ultrafast charge-transfer dynamics based on a modified Fano theory applied to the analysis of the photon-energy dependence of spectator resonant Auger scattering. The charge-transfer time is obtained from analysis of the two-dimensional spectral map of the dispersive exciton feature. As an illustration we apply the derived method to extract the charge-transfer time from our experiment with core-excited Xe on a Au surface using the map of electron kinetic energies in the L${}_3$M${}_{4,5}$M${}_{4,5}$ Auger region across the L${}_3$ resonance. The method developed provides an effective energy-domain alternative to rather challenging direct time-resolved measurements in the femto- and attosecond time domains.