How can I look up values which are indirectly linked to my dataframe?

I'd like some ideas/solutions for the following example. I have two tables, deals.df and turnover.df. Please see a toy example below which reflects what they look like:

deals.df <- data.frame(DealID = c(100101, 100102, 100103, 100104),
                       CompanyName= c('ABC', 'ABC', 'DEF', 'HIJ'),
                       DealYear = c(2013,2014,2015,2013),
                       DealYearL1= deals.df$DealYear-1,
                       DealYearP1= deals.df$DealYear+1,
                       DealYearL1Turnover= c('?', '?', '?', '?'),
                       DealYearTurnover= c('?', '?', '?', '?'),
                       DealYearP1Turnover= c('?', '?', '?', '?'))

turnover.df <- data.frame(CompanyName=c('ABC', 'DEF', 'HIJ'),
                          Turnover2011= c(100, 150, 180),
                          Turnover2012=c(110, 160, 200),
                          Turnover2013= c(125, 175, 210),
                          Turnover2014= c(135,180,230),
                          Turnover2015= c(145, 200, 235),
                          Turnover2016= c(160, 220, 250))

Note DealYearL1 means "Deal year less 1" and DealYearP1 means "Deal year plus one". What I want to do is populate the variables with "?". The logic is something like this:

For each DealID in deals.df, find the turnover associated with that CompanyName in turnover.df in the year before the deal, the year of the deal, and the year after the deal (ie the variables with '?' values). So for example the top row would look like this:

#Ideal output in deals.df

DealID   CompanyName   DealYear   DealYearL1   DealYearP1   DealYearL1Turnover   DealYearTurnover     DealYearP1Turnover
100101   ABC          2013         2012         2014           110                     125                135