We performed a detailed analysis of elemental abundances, dust features, and polycyclic aromatic hydrocarbons (PAHs) in the C-rich planetary nebula (PN) Wray16-423 in the Sagittarius dwarf spheroidal galaxy, based on a unique data set taken from the Subaru/HDS, MPG/ESO FEROS, HST/WFPC2, and Spitzer/IRS. We performed the first measurements of Kr, Fe, and recombination O abundance in this PN. The extremely small [Fe/H] implies that most Fe atoms are in the solid phase, considering into account the abundance of [Ar/H]. The Spitzer/IRS spectrum displays broad 16-24 {mu}m and 30 {mu}m features, as well as PAH bands at 6-9 and 10^-14^ {mu}m. The unidentified broad 16-24 {mu}m feature may not be related to iron sulphide (FeS), amorphous silicate, or PAHs. Using the spectral energy distribution model, we derived the luminosity and effective temperature of the central star, and the gas and dust masses. The observed elemental abundances and derived gas mass are in good agreement with asymptotic giant branch nucleosynthesis models for an initial mass of 1.90 M_{sun}_ and a metallicity of Z=0.004. We infer that respectively about 80, 50, and 90 per cent of the Mg, S, and Fe atoms are in the solid phase. We also assessed the maximum possible magnesium sulphide (MgS) and iron-rich sulphide (Fe50S) masses and tested whether these species can produce the band flux of the observed 30 {mu}m feature. Depending on what fraction of the sulphur is in sulphide molecules such as CS, we conclude that MgS and Fe50S could be possible carriers of the 30 {mu}m feature in this PN.